Modelling of Photon localization within THz Quantum Cascade Lasers

The terahertz (THZ) quantum cascaded laser is a successful example of electron wave function engineering with an active region comprising of repeated semiconductor quantum wells. In conventional QCL photon confinement is achieved using Fabry-Perot cavities formed by a transmitting mirror which gives a number of competing lasing modes resulting in poor spectral purity and leading to instability. However there are many applications that require a single mode operation. For this purpose waveguide of the QCL is periodically etched to force the laser to operate at a single frequency[1]. The periodic etching of the wave guide does not exhibit real space photon localization and also it will be more desirable to have a laser that can lase at multiple frequencies[2]. To achieve further control of the device photonic band properties, photonic bandgap engineering is required which introduces single or multiple defects into the photonic lattice. In this report, a single-defect photonic lattice is implemented in a chirped super-lattice THz DFB-Quantum cascade laser, aiming to show that the device output can be tuned efficiently from single-mode emission f1 to another single mode emission f2 by changing the applied bias[1]. Similarly introducing multiple defects in structures will enable the laser to give multi-mode lasing peaks at the output. Multiple defects photonic structures were implemented for the DFB-Quantum cascade laser in the project aiming to achieve three distinct lasing peaks that can be tuned by changing the applied bias. Gain was introduced in to these structures to enhance the power of the lasing peaks. It was found that introducing multiple defects into the photonic lattice for the DFB-QCL gave multi-mode lasing but the tuning was very sensitive to the applied bias. Therefore to tune the multi-mode lasing efficiently with applied bias further modelling is required. The work done in this project is basically scaling unchartered territory in the field of implementing aperiodic lattices for the active DFB-Quantum cascade laser device, whereby applied bias is used for the dual mode lasing operation.